Android应用程序的启动流程是什么

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应用进程的启动流程

本文基于Android 11，主要分析应用程序的启动流程，会直接定位到ActivityStackSupervisor.startSpecificActivity函数开始，因为该函数前面的内容主要在Activity的启动流程中，可以通过这部分的文章来阅读。

看源码流程，需要戒骄戒躁，心态好。配合源码使用，建议先收藏，夜深人静，心血来潮再看。

通过分析应用进程的启动流程，可以得到：

• 在Framework层，现在不止有AMS负责请求Zygote进程创建新进程，还有ATMS、ActivityStarter、ActivityTaskManger、ActivityTaskS在协助分担一些参数和逻辑的检查。

• 每个进程都是通过fork Zygote进程而来，且获得Java虚拟机。也就是说每一个应用进程都有自己的虚拟机。

• 应用进程是通过Soket去请求Zygote进程fork自己的。

• 每个进程都有自己的Binder线程池用于IPC。

• 每个应用进程的主线程在ActivityThread,其main函数会创建消息循环机制。

1、ActivityStackSupervisor.startSpecificActivity

ATMS有一个ProcessMap<WindowProcessController>类型的mProcessNames ,用于存储封装了已启动进程信息ProcessRecord和窗口信息Windows的WindowProcessController实例。WindowProcessController用于协调ActivityManger管理ProcessReocrd和WindwManger管理WIndow和Activity的关系。

void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
    // Is this activity's application already running?
    final WindowProcessController wpc =
            mService.getProcessController(r.processName, r.info.applicationInfo.uid);
    boolean knownToBeDead = false;
    if (wpc != null && wpc.hasThread()) {
        realStartActivityLocked(r, wpc, andResume, checkConfig);
        return;
        ...
        knownToBeDead = true;
    }
    r.notifyUnknownVisibilityLaunchedForKeyguardTransition();
    final boolean isTop = andResume && r.isTopRunningActivity();
    mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
}

这里的mService是ActivityTaskManagerService的实例，通过getProcessController函数获得当前wpc对象，判断当前启动应用进程是否启动wpc != null && wpc.hasThread(),如果条件成立，则开始真正启动一个未启动过的Activity,通过realStartActivityLocked；条件不成立，则调用mService的startProcessAsync启动当前Activity的所在的进程。即startSpecificActivity函数是启动进程和启动Activity的一个分界点。

2、ATMS.startProcessAsync

PooledLambda.obtainMessage函数是Lambda的调用方式，表示调用ActivityManagerInternal的startProcess函数，后续则是其参数。并返回一个Message对象，发给Handler类型的mH。

void startProcessAsync(ActivityRecord activity, boolean knownToBeDead, boolean isTop,
        String hostingType) {
    final Message m = PooledLambda.obtainMessage(ActivityManagerInternal::startProcess,
            mAmInternal, activity.processName, activity.info.applicationInfo, knownToBeDead,
            isTop, hostingType, activity.intent.getComponent());
    mH.sendMessage(m);
}

抽象类ActivityManagerInternal的继承类定义在ActivityManagerService的内部类LocalService。

public final class LocalService extends ActivityManagerInternal

3、LocalService.startProcess

@Override
public void startProcess(String processName, ApplicationInfo info, boolean knownToBeDead,
        boolean isTop, String hostingType, ComponentName hostingName) {
    startProcessLocked(processName, info, knownToBeDead, 0 /* intentFlags */,
            new HostingRecord(hostingType, hostingName, isTop),
            ZYGOTE_POLICY_FLAG_LATENCY_SENSITIVE, false /* allowWhileBooting */,
            false /* isolated */, true /* keepIfLarge */);  
}

4、startProcessLocked函数

final ProcessRecord startProcessLocked(String processName,
        ApplicationInfo info, boolean knownToBeDead, int intentFlags,
        HostingRecord hostingRecord, int zygotePolicyFlags, boolean allowWhileBooting,
        boolean isolated, boolean keepIfLarge) {
    return mProcessList.startProcessLocked(processName, info, knownToBeDead, intentFlags,
            hostingRecord, zygotePolicyFlags, allowWhileBooting, isolated, 0 /* isolatedUid */,
            keepIfLarge, null /* ABI override */, null /* entryPoint */,
            null /* entryPointArgs */, null /* crashHandler */);
}

5、ProcessList.startProcessLocked

ProcessList类的startProcessLocked函数，有几个重载函数，第一个调用。

在 ！isolated，判断了启动IntentFlag是否后台运行，是的话，直接拒绝。否则清理AMS中发生过Crash的进程（当前应用）。

分析一：创立当前应用进程的描述ProcessRecord。

判断当前系统是否启动完毕，未启动完毕，将进程信息缓存到AMS的mProcessesOnHold中。

分析二：调用了另外一个重载函数。

    final ProcessRecord startProcessLocked(String processName, ApplicationInfo info,
            boolean knownToBeDead, int intentFlags, HostingRecord hostingRecord,
            int zygotePolicyFlags, boolean allowWhileBooting, boolean isolated, int isolatedUid,
            boolean keepIfLarge, String abiOverride, String entryPoint, String[] entryPointArgs,
            Runnable crashHandler) {
        long startTime = SystemClock.uptimeMillis();
        ProcessRecord app;
		//isolated传递进来是false，
        if (!isolated) {
			//从mProcessNames缓存获取，由于是首次创建，null
            app = getProcessRecordLocked(processName, info.uid, keepIfLarge);
            checkSlow(startTime, "startProcess: after getProcessRecord");
			//判断要启动进程是否后台运行，直接return null
            if ((intentFlags & Intent.FLAG_FROM_BACKGROUND) != 0) {
                if (mService.mAppErrors.isBadProcessLocked(info)) {
                    return null;
                }
            } else {
                //重置进程的crash状态,使其处于正常状态
                mService.mAppErrors.resetProcessCrashTimeLocked(info);
                if (mService.mAppErrors.isBadProcessLocked(info)) {
                    mService.mAppErrors.clearBadProcessLocked(info);
                    if (app != null) {
                        app.bad = false;
                    }
                }
            }
        } else {
            app = null;
        }
        ProcessRecord precedence = null;
        if (app != null && app.pid > 0) {
            if ((!knownToBeDead && !app.killed) || app.thread == null) {
                app.addPackage(info.packageName, info.longVersionCode, mService.mProcessStats);
                return app;
            }
            ProcessList.killProcessGroup(app.uid, app.pid);
            precedence = app;
            app = null;
        }
        if (app == null) {
			// 分析一、创建新的应用进程描述ProcessRocrd
			//内部会将自己添加到mProcessNames中
			app = newProcessRecordLocked(info, processName, isolated, isolatedUid, hostingRecord);
			if (app == null) {
                return null;
            }
			//此时三者都是null
            app.crashHandler = crashHandler;
            app.isolatedEntryPoint = entryPoint;
            app.isolatedEntryPointArgs = entryPointArgs;
            if (precedence != null) {
                app.mPrecedence = precedence;
                precedence.mSuccessor = app;
            }
        } else {
            app.addPackage(info.packageName, info.longVersionCode, mService.mProcessStats);
        }
        // If the system is not ready yet, then hold off on starting this
        // process until it is.
        if (!mService.mProcessesReady
                && !mService.isAllowedWhileBooting(info)
                && !allowWhileBooting) {
            if (!mService.mProcessesOnHold.contains(app)) {
                mService.mProcessesOnHold.add(app);
            }
            if (DEBUG_PROCESSES) Slog.v(TAG_PROCESSES,
                    "System not ready, putting on hold: " + app);
            checkSlow(startTime, "startProcess: returning with proc on hold");
            return app;
        }
		分析二：
        final boolean success =
                startProcessLocked(app, hostingRecord, zygotePolicyFlags, abiOverride);
        checkSlow(startTime, "startProcess: done starting proc!");
        return success ? app : null;
    }

6、ProcessList.startProcessLocked重载

再次调用另外一个重载函数。

final boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
        int zygotePolicyFlags, String abiOverride) {
    return startProcessLocked(app, hostingRecord, zygotePolicyFlags,
            false /* disableHiddenApiChecks */, false /* disableTestApiChecks */,
            false /* mountExtStorageFull */, abiOverride);
}

重载函数,这个重载函数处理逻辑很长，主要给前面创建的ProcessRecord类型的app设置各种属性。例如外部存储挂载模式，应用进程运行模式，abi架构等等，其中包括最重要一点就是分析一，确定要启动进程的的类名：android.app.ActivityThread。分析二，继续调用重载函数。

    boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
            int zygotePolicyFlags, boolean disableHiddenApiChecks, boolean disableTestApiChecks,
            boolean mountExtStorageFull, String abiOverride) {
			...
            app.gids = gids;
            app.setRequiredAbi(requiredAbi);
            app.instructionSet = instructionSet;
            final String seInfo = app.info.seInfo
                    + (TextUtils.isEmpty(app.info.seInfoUser) ? "" : app.info.seInfoUser);
            //分析一：确定要启动应用程序的类名
            final String entryPoint = "android.app.ActivityThread";
			//分析二：调用另外一个重载函数
            return startProcessLocked(hostingRecord, entryPoint, app, uid, gids,
                    runtimeFlags, zygotePolicyFlags, mountExternal, seInfo, requiredAbi,
                    instructionSet, invokeWith, startTime);
        } catch (RuntimeException e) {
		  	...
        }
    }

重载函数：也是设置一些属性，然后调用startProcess函数。

 boolean startProcessLocked(HostingRecord hostingRecord, String entryPoint, ProcessRecord app,
            int uid, int[] gids, int runtimeFlags, int zygotePolicyFlags, int mountExternal,
            String seInfo, String requiredAbi, String instructionSet, String invokeWith,
            long startTime) {
            ...
            final Process.ProcessStartResult startResult = startProcess(hostingRecord,
                    entryPoint, app,
                    uid, gids, runtimeFlags, zygotePolicyFlags, mountExternal, seInfo,
                    requiredAbi, instructionSet, invokeWith, startTime);
            handleProcessStartedLocked(app, startResult.pid, startResult.usingWrapper,
                    startSeq, false);
           ...
        }
    }

7、ProcessList.startProcess

ProcessList类的startProcess函数会根据hostingRecord属性mHostingZygote判断走不同的创建分支，前面创建使用默认值，所以走了else分支。通过 Process.start函数创建新的应用进程。

Process.start的一路调用:

Process.start=>ZygoteProcess.start=>ZygoteState.start=>ZygoteState.startViaZygote

8、ZygoteState.startViaZygote

startViaZygote函数，主要是将传递进来的参数拼接成成字符串和收集起来。其中processClass是

private Process.ProcessStartResult startViaZygote(...)
                                                  throws ZygoteStartFailedEx {
        //根据传递进来的参数，拼接成字符串并收集到ArrayList<String>类型argsForZygote
        //将作为新应用程序的主函数的参数
        return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi),
                                          zygotePolicyFlags,
                                          argsForZygote);
}

9、ZygoteState.openZygoteSocketIfNeeded

zygoteSendArgsAndGetResult的第一个参数，调用了openZygoteSocketIfNeeded函数。尝试建立与Socket的连接（如果之前未建立的话）。我们知道Zygote进程在创建的过程，会调用runSelectLoop函数，创建Server端的Socket,一直等待来自AMS的Client端的Socket创建进程请求。

private ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
    try {
    	//建立和Zygote的Socket连接
        attemptConnectionToPrimaryZygote();
		//匹配abi的架构。在Zygote的创建对应四种模式:32,32_64和64，64_32
		//32,64
        if (primaryZygoteState.matches(abi)) {
            return primaryZygoteState;
        }
		//主要架构模式不配，匹配第二种 32_64,64_32
        if (mZygoteSecondarySocketAddress != null) {
            // The primary zygote didn't match. Try the secondary.
            attemptConnectionToSecondaryZygote();
            if (secondaryZygoteState.matches(abi)) {
                return secondaryZygoteState;
            }
        }
    } catch (IOException ioe) {
        throw new ZygoteStartFailedEx("Error connecting to zygote", ioe);
    }
    throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);
}

attemptConnectionToPrimaryZygote函数主要通过底层的LocalSocket创建与Zygote进程的Socket连接，并获得输入流zygoteInputStream和输出流zygoteOutputWriter。

private void attemptConnectionToPrimaryZygote() throws IOException {
    if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
        primaryZygoteState =
                ZygoteState.connect(mZygoteSocketAddress, mUsapPoolSocketAddress);
        maybeSetApiBlacklistExemptions(primaryZygoteState, false);
        maybeSetHiddenApiAccessLogSampleRate(primaryZygoteState);
    }
}

和Zygote进程的Server端Socket建立连接后，就是开始往Socket写数据了。

10、attemptZygoteSendArgsAndGetResult

回到第8步调用了zygoteSendArgsAndGetResult函数，又调用了attemptZygoteSendArgsAndGetResult函数。

zygoteSendArgsAndGetResult=>attemptZygoteSendArgsAndGetResult

11、attemptZygoteSendArgsAndGetResult

到这里，通过Socket的方式向Zygote进程写进前面拼接好的参数，Zygote在Server端的Socket接收到数据之后，会执行创建动作。在返回的result.pid>=0表示创建成功，并运行在新的进程。

    private Process.ProcessStartResult attemptZygoteSendArgsAndGetResult(
            ZygoteState zygoteState, String msgStr) throws ZygoteStartFailedEx {
        try {
            final BufferedWriter zygoteWriter = zygoteState.mZygoteOutputWriter;
            final DataInputStream zygoteInputStream = zygoteState.mZygoteInputStream;
            zygoteWriter.write(msgStr);
            zygoteWriter.flush();
            Process.ProcessStartResult result = new Process.ProcessStartResult();
            result.pid = zygoteInputStream.readInt();
            result.usingWrapper = zygoteInputStream.readBoolean();
            if (result.pid < 0) {
                throw new ZygoteStartFailedEx("fork() failed");
            }
            return result;
        } catch (IOException ex) {
            zygoteState.close();
            Log.e(LOG_TAG, "IO Exception while communicating with Zygote - "
                    + ex.toString());
            throw new ZygoteStartFailedEx(ex);
        }
    }

12、Zygote.main

在Zygote的启动流程过程，调用了ZygoteInit的main函数，因为Zygote是通过fork自身来创建其他进程，所以需要根据传递进来的参数，进行判断是启动什么类型的进程，例如自身isPrimaryZygote=true，或者SystemServer进程。然后通过ZygoteServer.runSelectLoop函数，等待其他进程请求创建新的进程。

    public static void main(String argv[]) {
        ZygoteServer zygoteServer = null;
        Runnable caller;
        try {
        	...
            boolean startSystemServer = false;
            String zygoteSocketName = "zygote";
            String abiList = null;
            boolean enableLazyPreload = false;
            for (int i = 1; i < argv.length; i++) {
                if ("start-system-server".equals(argv[i])) {
                    startSystemServer = true; //判断是否SystemServer进程
                } else if ("--enable-lazy-preload".equals(argv[i])) {
                    enableLazyPreload = true;
                } else if (argv[i].startsWith(ABI_LIST_ARG)) {
                    abiList = argv[i].substring(ABI_LIST_ARG.length());
                } else if (argv[i].startsWith(SOCKET_NAME_ARG)) {
                	//SCOKET_NAME_ARG="--socket-name=",根据参数得到SocketName
                    zygoteSocketName = argv[i].substring(SOCKET_NAME_ARG.length());
                } else {
                    throw new RuntimeException("Unknown command line argument: " + argv[i]);
                }
            }
		    //PRIMARY_SOCKET_NAME=zygote
            final boolean isPrimaryZygote = zygoteSocketName.equals(Zygote.PRIMARY_SOCKET_NAME);
         	gcAndFinalize();
            Zygote.initNativeState(isPrimaryZygote);
            ZygoteHooks.stopZygoteNoThreadCreation();
            zygoteServer = new ZygoteServer(isPrimaryZygote);
            if (startSystemServer) {
            	//启动SystemServer进程
                Runnable r = forkSystemServer(abiList, zygoteSocketName, zygoteServer);
                if (r != null) {
                    r.run();
                    return;
                }
            }
		   //循环等待AMS来请求创建新的进程
            caller = zygoteServer.runSelectLoop(abiList);
        } catch (Throwable ex) {
            Log.e(TAG, "System zygote died with exception", ex);
            throw ex;
        } finally {
            if (zygoteServer != null) {
                zygoteServer.closeServerSocket();
            }
        }
		//调用新的进程主函数
        if (caller != null) {
            caller.run();
        }
    }

13、ZygoteServer.runSelectLoo

这里只关注ZygoteServer.runSelectLoop函数，接受Socket客户端数据。

 /**
     * Runs the zygote process's select loop. Accepts new connections as
     * they happen, and reads commands from connections one spawn-request's
     * worth at a time.
     */
    Runnable runSelectLoop(String abiList) {
        while (true) {
         	...
            ZygoteConnection connection = peers.get(pollIndex);
            final Runnable command = connection.processOneCommand(this);
			...
            if (mIsForkChild) {
                return command;
            }
            ....       
   		}
	}

14、ZygoteConnection.processOneCommand

runSelctLoop主要是从循环中检测是否有连接建立，建立之后执行ZygoteConnection的processOneCommand函数，并返回一个Runable类型的command对象。

Runnable processOneCommand(ZygoteServer zygoteServer) {
    ...
    args = Zygote.readArgumentList(mSocketReader);
    //根据参数内容，作其他类型的处理
    ...
    //创建进程,调用底层nativeForkAndSpecialize方法，通过fork当前进程来创建一个子线程。
    pid = Zygote.forkAndSpecialize(parsedArgs.mUid, parsedArgs.mGid, parsedArgs.mGids,
            parsedArgs.mRuntimeFlags, rlimits, parsedArgs.mMountExternal, parsedArgs.mSeInfo,
            parsedArgs.mNiceName, fdsToClose, fdsToIgnore, parsedArgs.mStartChildZygote,
            parsedArgs.mInstructionSet, parsedArgs.mAppDataDir, parsedArgs.mIsTopApp,
            parsedArgs.mPkgDataInfoList, parsedArgs.mWhitelistedDataInfoList,
            parsedArgs.mBindMountAppDataDirs, parsedArgs.mBindMountAppStorageDirs);
    ...
    if (pid == 0) {
        //设置mIsForkChild=true
        zygoteServer.setForkChild();
       //关闭Socket连接
        zygoteServer.closeServerSocket();
        IoUtils.closeQuietly(serverPipeFd);
        serverPipeFd = null;
       //执行子进程内容
        return handleChildProc(parsedArgs, childPipeFd, parsedArgs.mStartChildZygote);
    } 
    ...
}

15、handleChildProc

handleChildProc函数。

private Runnable handleChildProc(ZygoteArguments parsedArgs,
        FileDescriptor pipeFd, boolean isZygote) {
        ...
        if (!isZygote) {
            return ZygoteInit.zygoteInit(parsedArgs.mTargetSdkVersion,
                    parsedArgs.mDisabledCompatChanges,
                    parsedArgs.mRemainingArgs, null /* classLoader */);
        } else {
            return ZygoteInit.childZygoteInit(parsedArgs.mTargetSdkVersion,
                    parsedArgs.mRemainingArgs, null /* classLoader */);
        }
}

16、 ZygoteInit.zygoteInit

public static final Runnable zygoteInit(int targetSdkVersion, long[] disabledCompatChanges,
        String[] argv, ClassLoader classLoader) {
    RuntimeInit.commonInit();
    ZygoteInit.nativeZygoteInit();//为新进程创建Binder线程池
    return RuntimeInit.applicationInit(targetSdkVersion, disabledCompatChanges, argv,
            classLoader);
}

以前还以为每个进程共用一个Binder线程池，现在知道每个进程都有自己的Binder线程池进行IPC。

17、RuntimeInit.applicationInit

    protected static Runnable applicationInit(int targetSdkVersion, long[] disabledCompatChanges,
            String[] argv, ClassLoader classLoader) {
        final Arguments args = new Arguments(argv); 
        return findStaticMain(args.startClass, args.startArgs, classLoader);
    }

这里的args.startClass就是Socket客户端传递下来的android.app.ActivityThread。

18、RuntimeInit.findStaticMain

RuntimeInit.findStaticMain函数主要通过反射创建ActivityThread类的实例，并反射主函数main，然后封装到MethodAndArgsCaller实例中返回。

protected static Runnable findStaticMain(String className, String[] argv,
        ClassLoader classLoader) {
    ...
    Class<?> cl = Class.forName(className, true, classLoader);
    Method m = cl.getMethod("main", new Class[] {						
						
						
						
						
						
						
					

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